1. Field of the Invention
The present invention relates to fiber reinforced resin composites. In particular, the invention relates to the use of a partially impregnated preform comprising a layer of continuous fiber material having a partially impregnated resin on one or both faces of the fiber material which forms a monolithic composite upon curing.
2. Related Background Art
In recent years the use of high-strength-to-weight ratio fiber reinforced resin composites has continuously expanded, particularly in weight-sensitive products, such as aircraft and space vehicles. Fiber reinforced resin composites used in such products have usually been created by forming a layup, e.g., a stack of layers or plies, the layers or plies being formed of unidirectional or multidirectional (e.g., woven) fabrics made of glass or graphite fibers completely preimpregnated with a resin. Such plies preimpregnated with resin are commonly referred to as "prepreg" plies or simply "prepreg". Normally the layup comprising the stack of layers or plies is positioned atop a forming tool, which, in its simplest form, may comprise a flat plate. After the layup is prepared, heat and pressure are applied. The heat cures the resin and the pressure compresses the layup preventing air and other gases, including volatile gases, from forming pores (bubbles) as the resin cures. Normally, an autoclave is used to apply the necessary heat and pressure.
While monolithic structures formed of fiber reinforced resin composites processed in the manner described above are satisfactory in some environments, they have certain disadvantages. For example, it has become desirable to provide cross-ply reinforcement in order to increase resistance to "in-plane" compression load failure, particularly after limited input damage. In-plane loads are those loads lying in the plane of the plies. Cross-ply reinforcement (sometimes referred to as Z-direction reinforcement) is created by cross-ply stitching a layup. However, the implementation of cross-ply stitching has proven to be difficult to accomplish. The difficulty occurs because the prepregs are preimpregnated with resin, which is tacky. The resin makes it extremely difficult to cross-ply stitch the fiber layers together. The needle becomes contaminated or otherwise gummed up with the tacky resin making it extremely difficult to stitch the layers together. The needle also causes damage to the fibers in the layers.
Another disadvantage of using fiber plies preimpregnated with resin is the difficulty of removing gases trapped between the plies when a layup is formed and the gases are produced in the layup when the resin is being cured. While the pressure applied during curing forces most entrapped gases into solution, some bubbles still form, resulting in formation of weakening voids in the resultant monolithic structure.
A further disadvantage associated with the use of preimpregnated fiber plies is the need to store such plies at a low temperature and the losses associated with the failure to use such plies in a timely manner. More specifically, as will readily be appreciated by those familiar with resins used to date to create fiber reinforced resin composites, the rate of resin curing is accelerated when resin temperature is raised. Conversely, the rate of resin curing is retarded by low temperatures. As a result, conventionally, prior to use, preimpregnated fiber plies (which are usually in the form of relatively wide tape or fabric on rolls prior to being laid up) are stored in a refrigerated environment. Since the low storage temperature impedes resin curing, the usable life of preimpregnated fiber plies is increased. However, even at low temperatures resins may cure, albeit at a slower rate. As a result, at some point, even preimpregnated fiber plies stored at low temperature become unusable and must be disposed of. Even though the resin is the only portion of the preimpregnated fiber ply that becomes useless, the fiber as well as the resin must be disposed of because the resin has started to cure.
In the case of the production of a composite aircraft wing structure, damage-tolerance of the composite wing structure is enhanced by stitching together the fabric layers used to form the composite structure. In current prior art processes, stitching of the fabric layers must occur prior to resin preimpregnation of the fabric because the needle used to stitch conventional prepregs causes excessive damage to the resin impregnated fibers. In order to meet this problem, the desired number of fabric layers are stitched in the absence of the resin and then, during the final curing process, resin is forced through the entire thickness of the prestitched fabric layers using a resin film infusion ("RFI") process. However, this approach leads to another problem. The resin must reach or infuse sufficiently to impregnate tall stiffeners in the wing structure to form a strong composite structure. Because it is very difficult to achieve full resin penetration to the vicinity of such stiffeners using these processes, it has been found that many anomalies exist in the resulting composite material.
A method and apparatus for creating monolithic structures formed of fiber reinforced resin composites, i.e., layers or plies bonded together by a cured resin is disclosed in U.S. Pat. No. 4,622,091. A plurality of dry plies are stacked to create a dry preform. The plies may or may not be stitched in the cross-ply direction. To form a composite a stack of dry preforms is created. After the stack is created, one or more layers of liquid or solid resin are added. The stack and the resin layer(s) are then cured under vacuum.
This method of creating monolithic structures suffers from the disadvantage that the plurality of dry plies must be stitched in the absence of the resin material. After stacking a plurality of stitched preforms to form a composite structure, the resin must then be hand placed between adjacent stitched dry preforms prior to infusion. This leads to increased manufacturing costs and production times.
The present invention is directed to avoiding the disadvantages of creating monolithic structures from preimpregnated fibrous layers with resins that require refrigeration or are not readily storable. More specifically, the invention is directed to a partially impregnated preform material that comprises a fabric layer partially impregnated with a resin which is stable against premature curing over long periods of time when stored at low temperatures. The partially impregnated preform is also stable at ambient temperatures when stored for shorter periods of time. The invention is also directed to fiber reinforced resin composites that are made from the instant partially impregnated preforms or a stack of preforms that can be easily cross-ply stitched in the presence of a resin film and that are formed in a manner that substantially reduces if not entirely eliminates weakening voids created by trapped gases by removing such gases prior to and during the infusion of the resin. Furthermore, the invention is directed to a process for preparing fiber reinforced resin composites that substantially reduces the amount of waste resulting from the premature curing of stored resins and the rejection of partially impregnated preforms due to poor quality. The invention is also directed to novel resin materials used in the inventive partially impregnated preforms.